The present invention relates to optical logic devices which perform logic with optical signals. More particularly, the present invention pertains to optical logic devices of reduced length which exchange AND and OR circuit function using one of the optical signals as a control signal. The present invention is also concerned with a light source for modulation suitably employed for a optical logic device and, more particularly, to a light source for modulation which is suitable for external modulation and which is integrated with a semiconductor laser, a semiconductor optical switch or a semiconductor light receiver for monitoring.
As is well known, the basic calculations performed in present electronic computers are 2 variable and 2 value logic in which 16 different kinds of result such as those shown in the truth table in FIG. 3 may be output with respect to values (0, 1) of two input signals a and b. This logic is used as the basic element when a more complicated logic is performed, and in such case all the complicated logic can be performed using the AND, OR and NOT basic logic. Accordingly, to arrange an optical computer which performs optical logic with optical signals, it is essential to realize an optical logic device which is capable of performing the above-described logic. However, a typical conventional optical logic device is arranged such that one light-emitting element and an optical switch are combined in such a manner as to obtain logic outputs of EX-NOR and EX-OR on the basis of two electrical signals, and no consideration has heretofore been taken for the technique of obtaining the logic outputs of AND and OR, which are basics of more complicated logic, by means of devices having the same arrangement.
Optical switches which have heretofore been proposed include one type which employs the deflection of light using the acoustooptic effect and/or electro-optic effect and another type in which the coupling coefficient of a directional coupler is changed using the electrooptic effect. Examples of materials which have large acoustooptic effect and/or electrooptic effect include LiNbO.sub.3 and PLZT. These materials are, however, different in terms of the crystal system from a compound semiconductor which constitutes a laser or a light receiver. It has, therefore, heretofore been impossible to form such material on the same substrate together with the above-described device in a monolithic structure, and it is necessary, when employing an external modulator, to first collimate the light emerging from a semiconductor laser and then allow the light to enter the external modulator, which involves troublesome and time-consuming operations such as alignment of optical axes and fixing optical elements. In addition, it is necessary to cope with the reflection of light from the external modulator and an optical element for collimating the light. In order to solve these problems, integration of an absorption type external modulator has already been tried. However, this integrated external modulator undesirably changes wavelength. As a part of formation of an integrated circuit, a technique of fabricating a laser and a light-receiving element for monitoring light as a signal in a monolithic structure has been examined. When the absorption type modulator is integrated, it is necessary to separate modulated light by means of an optical system when monitoring a modulated signal.
The present invention provides an optical logic device which comprises: an optical switch having two input ends and two output ends and adapted to allow at least the light entering one input end to emerge from any desired one of the output ends through one of the two optical guides which are switched over from one to the other by changing the refractive index at the intersection region between the optical guides in response to an applied signal; and two light-emitting elements which are respectively coupled to the two input ends, whereby AND and OR circuits are selectively formed. More specifically, referring to FIG. 4 which shows the fundamental concept of the present invention, when an optical switch 3 is OFF, the light which is emitted from a light-emitting element 1 and enters an optical guide 6 is allowed to emerge from an output end 5 through an optical guide 9, whereas the light which is emitted from a light-emitting element 2 and enters an optical guide 7 is allowed to emerge from an output end 4 through an optical guide 8. When the optical switch 3 is ON, the light from the light-emitting element 1 which enters the optical guide 6 is allowed to emerge from the output end 4 through the optical guide 8, whereas the light from the light-emitting element 2 which enters the optical guide 7 is allowed to emerge from the output end 5 through the optical guide 9. In addition, the light-emitting elements 1 and 2 are respectively represented by L.sub.1 and L.sub.2, while the ON state of the optical switch 3, which is denoted by S, is represented by "1", and the OFF state of the switch 3 is represented by "0". The output ends 4 and 5 are respectively represented by P.sub.1 and P.sub.2, and when light emerges from either P.sub.1 or P.sub.2, this state is represented by "1", whereas, when no light emerges therefrom, this state is represented by "0". FIGS. 5 and 6 are truth tables showing the above-described various states. Thus, when L.sub.2 is OFF, an AND logic output defined by a logic signal guided by L.sub.1. and S is obtained from P.sub.1, whereas, when L.sub.2 is ON, an OR logic output represented by a logic signal guided by L.sub.1 and S is obtained from P.sub.2.
In the prior art, since a material for an optical switch is different in terms of the crystal system from a compound semiconductor which is a material for a semiconductor laser, it has heretofore been impossible to integrate together a semiconductor laser and an optical switch, and an optical switch and a monitoring light receiver could not be formed in a monolithic integrated structure. In view of these circumstances, we have developed an optical switch recognizing the fact that the supply of current through a compound semiconductor causes a change in refractive index in the vicinity of the absorption wavelength due to a transition between bands. Thus, it becomes possible to fabricate a semiconductor laser and an optical switch from compound semiconductors in the same system. When an optical switch, a semiconductor laser and a light receiver are actually formed in a monolithic structure using an InP crystal as a substrate and an InGaAsP crystal as an active layer or an optical guide, the following advantages are obtained. Since the optical guide is formed in the same process and at the same time as the above-described optical elements, an advantageously high coupling efficiency is obtained in contrast to hybrid optical coupling, and since there is no boundary between different materials, the problem of reflection is eliminated, and it is possible to obtain reliability with respect to changes in external environments. It has heretofore been difficult, when effecting external modulation, to monitor a modulated signal and set a light-receiving system because the light emerging from a semiconductor laser in the opposite direction is not modulated. However, it becomes possible to effect monitoring easily by employing an optical switch as a modulator and by forming the optical switch from a material in the same crystal system as a material for a semiconductor laser and a light receiver with a different forbidden band amount, i.e. bandgap energy. In addition, since the optical switch according to the present invention has two input ends, if semiconductor lasers are respectively fabricated for these two input ends, even when one semiconductor laser fails, it can be replaced with the other, which means that it is possible to improve the yield of the device as a modulation light source and obtain redundancy of the light source of the system. Two lasers can be coupled to one optical switch in the same step in which one laser is fabricated in the process of forming an optical integrated circuit.
As described above, the optical logic device according to the present invention comprises: an optical switch having two input ends and two output ends and adapted to allow at least the light entering one input end to emerge from any desired one of the output ends by switching the output ends from one to the other in response to an applied signal; and two light-emitting elements which are respectively coupled to the two input ends. Thus, it is possible to obtain an optical logic device which has a reduced size, i.e., a length of 1 mm or less, and which can perform both AND and OR functions which are basic logics functions, so that it is possible to simply the arrangement of an optical logic and optical information processing system.
According to the present invention, the following various advantages are obtained.
Since the external modulator is fabricated on the same substrate together with the light-emitting elements, (1) there are no reflecting end faces, and (2) it is possible to allow the light emitted from the light-emitting elements to enter the external modulator with an increased efficiency. In addition, the reflection of light from the input ends of the external modulator is prevented, and it is therefore possible to eliminate one of the causes of feedback noise due to reflection.
The present invention enables a monitoring light receiver to be readily coupled to the external modulator.
The present invention further enables two lasers to be readily produced and coupled to the optical modulator (optical switch). It is therefore possible to keep a relatively high production yield of modulation integrated circuits with respect to a relatively low production yield of laser. When two lasers are operated, it is possible to increase the degree of redundancy of the system.